It is conventional in the measurement of the concentration of solid particles or soot in the exhaust gases of internal combustion engines to dilute the exhaust gases since hot exhaust gases cannot be readily measured. The dilution ratio of the exhaust gases is established on the basis of measuring the ratio of a gaseous component of the undiluted exhaust gases and the same gaseous component in the diluted exhaust gases.
This method has the disadvantage that it is necessary to measure the quantities of gaseous components which is relatively expensive even though it is only the quantity of solid particles which is desired.